Interface Design with Conducting Polypyrrole Thin Film for Improving the Storage Property of Ni-Rich Materials

Abstract

Layered high nickel composites, such as LiNi0.9Co0.1O2, LiNi0.85Co0.1Al0.05O2 and LiNi0.8Co0.1Al0.1O2, are considered to be the most promising materials for hybrid electric vehicles, plus-in bybrid vehicles, and electric vehicles due to their large capacity, good C-rate capacity and long cycle life. However, the characteristics of rapidly moisture absorbing and residual alkaline substance such as Li2CO3 and LiOH limited their practical applications. Considered the long life of the cathode material is an important factor during its application, the storage property of Ni-rich materials has attracted much attention. It is obviously known that the Ni-rich composites easily absorbed H2O and CO2 after being exposed to air, and Li2CO3, LiOH and NiO-like species were identified to the major species formed on the surface. These impurities are believed to be responsible for storage loss. Meanwhile, attempts have been made to reduce the storage loss by two approaches, ie.: (1) decreasing the nickel content on the surface of the compound, such as coating, core-shell structural design and gradient structural design; (2)storing the materials in an atmosphere without H2O and CO2. As a result, a marked improvement in storage properties is achieved. In our study, surface design with conducting polypyrrole thin film, which is just like a capsule shell, can avoid the indirect contact with atmosphere and protect the core materials form corrosion causing by HF attacking. The PPy-coated cathode materials were prepared by the chemical polymerization with and p-toluenesulfonate as the dopant. H2O2 was chosen as the oxidant in order to avoid the introduction of other impurity ions. X-ray diffraction (XRD) was employed to identify the crystalline phase of the products. The surface species on the materials surface were identified through Fourier transform infrared spectrometry (FTIR), X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDXS). Transmission electron microscope (TEM) was used to identify the thickness of the thin film. To quantify the amount of the conducting thin film, thermogravimetric analysis (TGA) was carried on. Exposure test was carried out to determine the storage properties of pure and treatment composites. In order to measure the amount of residual alkaline substance on the surface, powders were soaked in water and the LiOH/Li2CO3were measured using titration method. All electrochemical properties of the composites were investigated by using coin cells (2016 type). The results indicates that the Ni-rich composites designed with conducting thin film on the surface can effectively improve their storage properties. The amount of residual alkaline substance on the surface of powders coated with conducting thin film are greatly reduced after being exposed in air, compared with un-treatment powders. The thin film on the surface builds a conductive network, which is beneficial to improve their electrochemical properties as well.